Degradation discrimination system of internal combustion engine exhaust gas purification system

ABSTRACT

A system for discriminating degradation of an exhaust purification system of an internal combustion engine having an adsorbent installed at a bypass exhaust gas passage branched from an exhaust pipe of the engine, which is opened by a switch-over valve at starting of the engine to introduce the exhaust gas such that the adsorbent adsorbs unburned HC in exhaust gas generated by the engine and is closed such that the adsorbent desorbs the adsorbed HC and the desorbed HC is thereafter recirculated at a position upstream of a catalyst. Based on the fact, due to the adsorption effect, the HC concentration at a downstream point is lower than that at an upstream point, in the system, the concentration of the HC introduced in the bypass exhaust gas passage in the adsorption mode is detected and compared with a threshold value and the degradation of the adsorbent is discriminated, thereby enabling to improve the discrimination accurately.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a degradation discrimination system of aninternal combustion engine exhaust gas purification system, moreparticularly to a system for discriminating whether an exhaust gaspurification system comprising an adsorbent for adsorbing unburnedcomponents including the hydrocarbons (HC) in the exhaust gas generatedby an internal combustion engine, has degraded or deteriorated.

2. Description of the Related Art

Internal combustion engines are ordinarily provided with a catalyst (athree-way catalytic converter) in the exhaust system which removes HC,NOx and CO components in the exhaust gas generated by the engine.However, when the catalyst is not activated, for example, at the time ofengine cold-starting, unburned components of the exhaust gas includingunburned HC are released immediately into the atmosphere.

For that reason, there has been proposed an exhaust gas purificationsystem which has an adsorbent made of a zeolite material or some similarmaterial installed in a bypass exhaust gas passage branched from theexhaust pipe at a location downstream of the catalyst, which merges intothe exhaust pipe at a downstream point and has a switch-over valve whichopens or closes the bypass exhaust gas passage. The switch-over valveopens the bypass exhaust gas passage when the engine is started tointroduce the exhaust gas such that the adsorbent adsorbs unburnedcomponents including the HC when the catalyst is not activated andcloses the bypass exhaust gas passage such that the adsorbent desorbsthe adsorbed component and the desorbed components are thereafterrecirculated at a position upstream of the catalyst after the catalysthas been activated.

Since a desired exhaust gas purification can not be achieved if anydegradation or abnormality arises in such an engine exhaust gaspurification system, Japanese Laid-Open Patent Application No. Hei 8(1996)-93,458, for example, proposes the technique to discriminatewhether any degradation or abnormality arises in the system. AnotherJapanese Laid-Open Patent Application, No. Hei 8 (1996)-218, 850proposes a similar technique.

Specifically, Japanese Laid-Open Patent Application No. Hei 8(1996)-93,458 proposes providing an HC sensor at a position downstreamof the bypass exhaust gas passage for detecting the HC concentration ofthe exhaust gas at that location. In this prior art, the HC sensordetects the HC concentration in the adsorption mode and in thedesorption mode respectively. The detected values are compared withpredetermined values and based on the result of the comparison, it isdiscriminated whether any trouble has occurred in a mechanical part suchas a switch-over valve.

In addition, this prior art proposes providing the HC sensor at arecirculation passage to determine the total amount of HC beingrecirculated, and by comparing the determined amount with apredetermined value, it discriminates whether a trouble has occurred ina mechanical part such as a valve for opening/closing the recirculationpassage.

Japanese Laid-Open Patent Application No. Hei 8 (1996)-218,850 proposesproviding O₂ sensors at a position upstream of the adsorbent, butdownstream of the catalyst and at a position upstream of therecirculation passage. In this prior art, a time lapse until therich/lean signals of both the sensors become equal to each other ismeasured and based on the measured time, it discriminates whether anyabnormality arises in the system such as at the adsorbent.

Specifically, based on the assumption that the air/fuel ratio in therecirculation passage becomes richer than that at a position downstreamof the adsorbent since the desorbed HC is recirculated together with theexhaust gas, but becomes equal to the air/fuel downstream of theadsorbent after the desorbed HC has been purified, the second prior artconducts the discrimination by measuring the time lapse until theoutputs of the air/fuel ratio sensors coincide. Similarly, the secondprior art proposes providing an HC sensor, instead of the oxygen sensor,to discriminate the occupance of abnormality based on the HCconcentration in the desorption mode.

Although the first prior art mentioned in Japanese Laid-Open PatentApplication No. Hei 8 (1996)-93, 458 monitors the behavior of HCdirectly through the HC sensor, the first prior art can not discriminatewhether the abnormality has occurred in the adsorbent or any other partsuch as switch-over valve. In other words, the first prior art can notdiscriminate the abnormality or degradation arising in the adsorbentaccurately.

The second prior art mentioned in Japanese Laid-Open Patent ApplicationNo. Hei 8 (1996)-218,850 can similarly discriminate the abnormality ordegradation arising in the system including the adsorbent by detectingthe HC concentration in the desorbtion mode. However, since the secondprior art does not detect the concentration or amount of HC flowing intoand adhering to the adsorbent in the adsorption mode, the second priorart is not always satisfactory in the discrimination accuracy if thereare variances in the amount of HC. The same argument will also beapplied in a case when the amount of recirculation fluctuates.

SUMMARY OF THE INVENTION

An object of the invention is therefore to provide a degradationdiscrimination system of an internal combustion engine exhaust gaspurification system having an adsorbent installed at a bypass exhaustgas passage branched from the exhaust pipe that is opened by aswitch-over valve at engine starting to introduce the exhaust gas suchthat the adsorbent adsorbs unburned HC components in the exhaust gasgenerated by the engine and is closed such that the adsorbent desorbsthe HC component and the desorbed HC component is thereafterrecirculated at a location upstream of a catalyst, which enables todiscriminate the degradation or deterioration of the adsorbentaccurately by directly monitoring the behavior of the HC componentincluding the adsorption mode during which the adsorbent adsorbs the HCcomponent through an HC concentration detecting means.

In order to achieve the object, there is provided a system fordiscriminating degradation of an exhaust purification system of aninternal combustion engine having an adsorbent installed at a bypassexhaust gas passage branched from an exhaust pipe of the engine andmerged into the exhaust pipe at a location downstream of the adsorbent,the bypass exhaust gas passage being opened by a switch-over valve atstarting of the engine to introduce the exhaust gas such that theadsorbent adsorbs unburned HC in exhaust gas generated by the engine andbeing closed such that the adsorbent desorbs the adsorbed HC and thedesorbed HC is thereafter recirculated at a position upstream of acatalyst, comprising: an HC sensor installed in the bypass exhaust gaspassage at least one of a first position at the adsorbent and a secondposition downstream of the adsorbent for detecting concentration of HCintroduced in the bypass exhaust gas passage in an adsorption mode wherethe adsorbent adsorbs HC; and adsorbent degradation discriminating meansfor comparing the detected concentration of HC with a threshold valueand for discriminating whether the adsorbent has degraded based on aresult of comparison.

BRIEF EXPLANATION OF THE DRAWINGS

The objects and advantages of the invention will become more apparentfrom the following descriptions and drawings, in which:

FIG. 1 is a schematic view showing the overall configuration of adegradation discrimination system of an internal combustion engineexhaust gas purification system according to a first embodiment of theinvention;

FIG. 2 is an enlarged sectional view of a switch-over valve illustratedin FIG. 1;

FIG. 3 is a sectional view taken along III—III of FIG. 2;

FIG. 4 is a sectional view taken along IV—IV of FIG. 1;

FIG. 5 is a chart showing the property of zeolites (to be used as amaterial for an adsorbent illustrated in FIG. 1) relative to HCcompounds in the exhaust gas generated by the engine;

FIG. 6 is a block diagram showing the details of an Electronic ControlUnit (ECU) illustrated in FIG. 1;

FIGS. 7A-7C are views showing the operation of the exhaust gaspurification system including the adsorbent illustrated in FIG. 1;

FIG. 8 is a time chart showing the operation of the degradationdiscrimination system of an internal combustion engine exhaust gaspurification system according to the first embodiment illustrated inFIG. 1;

FIG. 9 is a view, similar to FIG. 8, but showing a case in which theadsorbent is discriminated not to be degraded;

FIG. 10 is a view, similar to FIG. 8, but showing a case in which theadsorbent is discriminated to be degraded;

FIG. 11 is a flow chart similarly showing the operation of the systemillustrated in FIG. 1;

FIG. 12 is a first half of a flow chart showing the subroutine forestimating and determining temperatures referred to in the flow chart ofFIG. 11;

FIG. 13 is a second half of the flow chart showing the subroutine forestimating and determining temperatures referred to in the flow chart ofFIG. 11;

FIG. 14 is a graph showing characteristics of a basic value of anestimated aft-catalyst exhaust gas temperature referred to in the flowchart of FIG. 12;

FIG. 15 is a graph showing characteristics of a post-engine-startingexhaust gas temperature correction coefficient referred to in the flowchart of FIG. 12;

FIG. 16 is a graph showing characteristics of coefficients used in thedetermination of an estimated catalyst temperature referred to in theflow chart of FIG. 12;

FIG. 17 is a graph showing characteristics of an estimated aft-catalystexhaust gas temperature referred to in the flow chart of FIG. 12;

FIG. 18 is a graph showing characteristics of coefficients used in thedetermination of an estimated adsorbent temperature referred to in theflow chart of FIG. 12;

FIG. 19 is a graph showing characteristics of coefficients used in thedetermination of the estimated adsorbent temperature referred to in theflow chart of FIG. 12;

FIG. 20 is a flow chart showing the subroutine for determining anestimated aft-catalyst in-exhaust gas HC concentration referred to inthe flow chart of FIG. 11;

FIG. 21 is a graph showing characteristics of a basic value of theestimated aft-catalyst exhaust gas temperature referred to in the flowchart of FIG. 20;

FIG. 22 is a graph showing characteristics of an engine-warmupcorrection coefficient k.engtmp referred to in the flow chart of FIG.20;

FIG. 23 is a graph showing characteristics of a catalyst purificationrate referred to in the flow chart of FIG. 20;

FIG. 24 is a flow chart showing the subroutine for determining the HCconcentration referred to in the flow chart of FIG. 11;

FIG. 25 is a graph showing characteristics of the HC concentrationreferred to in the flow chart of FIG. 24;

FIG. 26 is a graph showing characteristics of times defining adegradation discrimination period referred to in the flow chart of FIG.11;

FIG. 27 is a flow chart showing the subroutine for determining anestimated adsorbed HC amount referred to in the flow chart of FIG. 11;

FIG. 28 is a graph showing a characteristic of an HC density correctioncoefficient referred to in the flow chart of FIG. 27;

FIG. 29 is a flow chart showing the subroutine for determining athreshold value to be used for discriminating degradation referred to inthe flow chart of FIG. 11;

FIG. 30 is a graph showing characteristics of a basic value of anadsorbent adsorption rate referred to in the flow chart of FIG. 29;

FIG. 31 is a graph showing characteristics of a correction coefficientof the adsorbent adsorption rate referred to in the flow chart of FIG.29;

FIG. 32 is a flow chart showing the subroutine for determining theadsorbent degradation referred to in the flow chart of FIG. 11;

FIG. 33 is a flow chart showing the subroutine for determining thecompletion of purging of desorbed HC referred to in the flow chart ofFIG. 11;

FIG. 34 is a graph showing characteristics of an EGR flow rate referredto in the flow chart of FIG. 33;

FIG. 35 is a graph showing characteristics of an HC density correctioncoefficient referred to in the flow chart of FIG. 33;

FIG. 36 is a view, partially similar to FIG. 1, but showing theconfiguration of the degradation discrimination system of an internalcombustion engine exhaust gas purification system according to a secondembodiment of this invention;

FIG. 37 is a view, partially similar to FIG. 1, but showing theconfiguration of the degradation discrimination system of an internalcombustion engine exhaust gas purification system according to a thirdembodiment of this invention; and

FIG. 38 is a view, partially similar to FIG. 1, but showing theconfiguration of the degradation discrimination system of an internalcombustion engine exhaust gas purification system according to a fourthembodiment of this invention.

PREFERRED EMBODIMENTS OF THE INVENTION

Embodiments of the invention will now be explained with reference to thedrawings.

FIG. 1 is a view schematically showing the overall configuration of adegradation discrimination system of an internal combustion engineexhaust gas purification system according to an embodiment of theinvention.

Reference numeral 10 in this figure designates an overhead cam (OHC)in-line four-cylinder internal combustion engine. Air drawn into an airintake pipe or passage 12 through an air cleaner (not shown) mountedseparately is supplied to the first to fourth cylinders (only one isshown) through a surge tank 16, an intake manifold 18 and two intakevalves 20 (only one is shown), while the flow thereof is adjusted by athrottle valve 14 and is supplied to the first to four cylinders of theengine 10. The throttle valve 14 is bypassed by a bypass 22 provided atthe air intake pipe 12. A valve (EACV) 24 comprised of anelectromagnetic solenoid valve is installed in the bypass 22 for closingthe same.

A fuel injector 26 is installed in the vicinity of the intake valves 20of each cylinder for injecting fuel for the cylinder concerned. Theinjected fuel mixes with the intake air to form an air-fuel mixture thatis supplied into a combustion chamber 28 and is compressed in thecompression stroke and is ignited by a spark plug (not shown). Theresulting combustion of the air-fuel mixture drives a piston 30downwards.

The exhaust gas produced by the combustion is discharged through twoexhaust valves 34 (only one is shown) into an exhaust manifold 36, fromwhere it passes through an exhaust pipe or passage 38 to a firstcatalyst (catalytic converter) 40 installed immediately below theexhaust manifold 36 and a second catalyst 42 comprising a first catalystbed 42 a and a second catalyst bed 42 b (all three-way catalyticconverters) where noxious components are removed therefrom before it isdischarged into the atmosphere via a vehicle rear assembly 46 includinga muffler and a tail pipe (neither shown).

The engine 10 is equipped with a variable valve timing mechanism 50(illustrated as “V/T” in FIG. 1). The variable valve timing mechanism 50switches the opening/closing timing of the intake and/or exhaust valvesbetween two types of timing characteristics in response to the enginespeed NE and the engine load (e.g. the manifold absolute pressure PBA),i.e. a characteristic for low engine speed and a characteristic for highengine speed. The characteristics include one of the two intake valvesbeing operated at a rest position.

The exhaust pipe 38 is connected to a chamber 54, cylindrical in shape,at a location downstream of the second catalyst 42. More specifically,the exhaust pipe 38 is branched off downstream of the second catalyst 42to form a branch 52. The branch 52 is connected to the chamber 54 whichis air-tightly connected to the exhaust pipe 38 to surround the same.With this passages for exhaust gas flow are formed; a main exhaust gaspassage 38 a passing through the inside of the exhaust pipe 38 and abypass exhaust gas passage 56 passing through the branch 52 and theinner space of the chamber 54.

A switch-over valve 60 is provided in the vicinity of the branchingpoint at the entrance of the chamber 54. FIG. 2 is an enlarged sectionalview of the switch-over valve 60 and FIG. 3 is a sectional view takenalong III—III of FIG. 2.

Explaining the switch-over valve 60 with reference to FIGS. 2 and 3, itcomprises a first valve disc 60 a which is greater in diameter than theexhaust pipe inner wall 38 b defining the main exhaust gas passage 38 a,and an arm 60 b in an inverted-C shape which connects the first valvedisc 60 b with a second valve disc 60 c which is greater than thediameter of the wall 52 a of the branch 52 defining the bypass exhaustgas passage 56. A stem 60 d is used to connect the second valve disc 60c to a shaft 60 e.

As shown in FIG. 1, the shaft 60 e is connected to a valve actuator 64.The valve actuator 64 has a conduit 66 which is connected to the airintake pipe 12 at a location downstream of the throttle valve 14. Anelectromagnetic solenoid valve (referred later as “TRPV” ) 68 isinstalled in the conduit 66, which opens the conduit 66 when energizedto introduce the negative pressure therein.

Explaining the valve operation more specifically with reference to FIG.2, the valve actuator 64 operates to rotate shaft 60 e in the positionshown by solid lines in the figure when the negative pressure isintroduced such that the first valve disc 60 a rests on a valve seat 60f to close the main exhaust gas passage 38 a (in other words, it opensthe bypass exhaust gas passage 56). On the other hand, when the TRPV 68is deenergized, the conduit 66 is open to the air. As a result, theshaft 60 e is returned to a position shown by phantom lines in thefigure by a return spring (not shown) such that the second valve disc 60c rests on a valve seat 60 g to close the bypass exhaust gas passage 56(in other words, it opens the main exhaust gas passage 38 a).

The second valve disc 60 c (and the first valve disc 60 a) can be at anyposition between those illustrated in FIG. 2 by solid lines and phantomlines, by regulating the amount of negative pressure introduced in theconduit by operating the TRPV 68 in such a way that the bypass exhaustgas passage 56 (and the main exhaust gas passage 38 a) is opened by aslight amount.

As shown in FIG. 2, the first and second valve discs 60 a, 60 c arefixed to the shaft 60 e at a predetermined angle θ in such a way, thatwhen the first valve disc 60 a closes the main exhaust gas passage 38 a,the second valve disc 60 c is lifted from the valve seat 60 g so as notto block the exhaust gas flowing into the bypass exhaust gas passage 56,while, when the second valve disc 60 c closes the bypass exhaust gaspassage 56, the first valve 60 a is lifted from the valve seat 60 f soas not to block the exhaust gas flowing into the main exhaust gaspassage 38 a.

Returning to the explanation of FIG. 1, an adsorbent (HC adsorbing meansor HC adsorbing catalyst) 74 is installed at the bypass exhaust gaspassage 56 in the chamber 54. The adsorbent 74 comprises a firstadsorbent bed 74 a (provided upstream, i.e. at a position closer to thebranch 52) and a second adsorbent bed 74 b (provided downstream, i.e. ata position closer to the vehicle rear assembly 46).

Specifically, as shown in FIG. 4, the chamber 54 is configured to becylindrical in cross section such that it completely encircles theexhaust pipe 38. More specifically, the adsorbent 74 is positioned closeto the exhaust pipe 38 in such a way that the temperature increase ofthe adsorbent 74 is promoted such that the adsorbed unburned componentis desorbed as quickly as possible and is recirculated into the engineintake.

The adsorbent 74 preferably comprises a porous material having a largesurface area such as zeolite (the general name of crystallinealuminosilicate or metallosilicate). The adsorbent made from zeolite hassmall pores or holes in it which are regular in shape and size. The poresizes are different for different zeolites.

A pore size of 0.2 nm approximately corresponds to the molecular size ofHC. The adsorbent made from zeolite adsorbs HC at a low temperature,less than 100° C. and desorbs the adsorbed HC at a higher temperature,ranging from 100 ° C. to 250° C. These temperatures are different fordifferent kinds of HC (number of carbons) and increase with increasingnumber of carbons. Moreover, these temperatures also vary depending onthe kind of zeolites.

The adsorption is classified into two types, i.e. a mechanicaladsorption caused by intermolecular attractions and a chemicaladsorption caused by chemical bonds The adsorption in the zeoliteadsorbent is mainly the mechanical one. In the mechanical adsorption,the kind (number of carbons) of HC to be adsorbed is determined by thepore size of zeolite constituting the adsorbent.

FIG. 5 is a chart showing the property of zeolites relative to HCcompounds in five kinds (number of carbons). In the figure, the symbolsindicate the ability of adsorption, i.e. ∘: excellent; Δ: medium; :poor.

Thus, of the various zeolites, an appropriate zeolite or a combinationof zeolites such as a combination of Ga-MFI and mordenite shouldpreferably be selected as the adsorbent. Although not shown, some HCcompounds such as methane (CH₄) will require other kind of zeoliteshaving finer pores.

The adsorbent 74 should be prefabricated from a mixture of selectedzeolite(s) and a catalyzer element in a honeycomb structure held in aspecially designed metal casing. The adsorbent 74 made from any kind ofzeolite exhibits an excellent heat proof (thermal stability) propertyand does not degrade or deteriorate under a high temperature if thetemperature is less than 1100° C. or thereabout. The marginaltemperature (beneath of which zeolite does not degrade) differs fordifferent zeolites. If different zeolites are combined to be used, thecombination will determine the marginal temperature.

Returning to the explanation of FIG. 1, the exhaust pipe 38 is providednear the end of the chamber 54 (close to the vehicle rear assembly 46)with four holes (confluence points) 76 which are circumferentiallylocated at intervals of 90 degrees. The bypass exhaust gas passage 56 isthus formed from the branch 52 and extends into chamber 54 via theadsorbent 74 up to the holes 76 where it merges into the main exhaustgas passage 38 a in the exhaust pipe 38.

The chamber 54 is connected, at or near the entrance, i.e., at aposition upstream of the adsorbent 74 and close to the branch 52, to anEGR conduit (passage) 82. The EGR conduit 82 is connected, at the otherend, to the air intake pipe 12 at a position downstream of the throttlevalve 14. The EGR conduit 82 is provided with an EGR control valve(electromagnetic solenoid valve) 84 which closes the conduit 82 whenmade ON (energized). A lift sensor 86 is provided in the vicinity of theEGR control valve 84 and generates a signal indicative of the amount oflift (stroke) named “lact” of the valve 84. The lift amount indicatesthe opening degree of the valve 84.

The exhaust gas purification system comprises the adsorbent 74, thebypass exhaust gas passage 56, the switch-over valve 60, the valveactuator 64, the holes 76, the EGR conduit 82, the EGR control valve 84,etc.

The ignition distributor (not shown) of the engine 10 is provided with acrank angle sensor 90 which generates a signal indicative of Top DeadCenter (TDC) of the piston 30 and a signal indicative of unit anglesdivided into smaller values. The engine 10 is further provided with athrottle position sensor 92 which generates a signal indicative of thedegree of opening θTH of the throttle valve 14, a manifold absolutepressure (MAP) sensor 94 which generates a signal indicative of theaforesaid manifold absolute pressure PBA of the intake manifolddownstream of the throttle valve 14 in terms of absolute value andindicative of the engine load, and a coolant temperature sensor 96installed in the vicinity of a coolant passage (not shown) of the enginewhich generates a signal indicative of the temperature TW of the enginecoolant.

Further, an universal air/fuel ratio sensor 98 (named “LAF sensor”) isprovided in the exhaust pipe 38 at or downstream of a confluence pointof the exhaust manifold 36 and upstream of the first catalyst 40, whereit generates a signal indicative of the oxygen concentration in theexhaust gas, as explained later. In addition, an O₂ sensor 100 isinstalled in the exhaust pipe 38 at a location between the firstcatalyst bed 42 a and the second catalyst bed 42 b, which generates anON/OFF signal each time the oxygen concentration in the exhaust gaschanges from rich to lean and vice versa.

Furthermore, an HC sensor 104 is installed at the bypass exhaust gaspassage 56 in the chamber 54, more precisely at the second bed adsorbentbed 74 b at a location close to its rear end (close to the vehicle rearassembly 46) and generates a signal indicative of the concentration ofHC (hereinafter referred to as “trs.hc”) in the exhaust gas flowing inthe bypass exhaust gas passage 56 and flowing into the adsorbent 74 atthat location. The sensor may be located, as shown in the figure by thephantom lines with the reference 104 a, at a position between the firstadsorbent bed 74 a and the second adsorbent bed 74 b, or may be located,as shown in the figure by the phantom lines with the reference 104 b, ata position downstream of the second adsorbent bed 74 b (closer to thevehicle rear assembly 46)

The HC sensor 104 is, for example, a threshold current type sensor madefrom a solid-electrolyte comprising some kinds of barium oxides, asdisclosed in Japanese Laid-Open Patent Application No. Hei 10(1998)-300, 718.

Furthermore, a valve timing sensor (not shown) is provided in ahydraulic pressure circuit (not shown) of the variable valve timingmechanism 50 and generates a signal indicating which characteristic isselected.

These output signals generated by the sensors are forwarded to anElectronic Control Unit (ECU) 114 comprising a microcomputer.

Details of the ECU 114 are shown in the block diagram of FIG. 6.

The output of the LAF sensor 98 is received by a first detection circuit116, where it is subjected to appropriate linearization processing forproducing an output characterized in that it varies linearly with theoxygen concentration of the exhaust gas over a broad range extendingfrom lean to rich. The output of the O₂ sensor 100 is input to a seconddetection circuit 118 which generates a switching signal indicating thatthe air/fuel ratio in the exhaust gas emitted from the engine 10 is richor lean with respect to the stoichiometric air/fuel ratio. The output ofthe HC sensor 104 is input to a third detection circuit 119 whichgenerates a signal indicative of the concentration of HC in the exhaustgas.

The output of these detection circuits 116, 118, 119 are forwardedthrough a multiplexer 120 and an AID converter 122 to a RAM (randomaccess memory) 124 in a CPU (central processing unit). Specifically, theCPU has a CPU core 130, a ROM (read-only memory) 132 and the RAM 124,and the output of the detection circuits 116, 118, 119 are A/D-convertedand stored in buffers of the RAM 124. The outputs of the analog sensorsincluding the throttle position sensor 92 are similarly input to the CPUvia the A/D converter 122 to the RAM 124.

The output of the crank angle sensor 90 is shaped by a wave-form shaper126 and has its output value counted by a counter 128. The count isinputted into the CPU and the engine speed NE is detected or calculatedfrom the count. In accordance with commands stored in the ROM 132, theCPU core 130 computes a manipulated variable including a basic fuelinjection amount TI and an output fuel injection amount TOUT obtained bycorrecting TI and drives the fuel injectors 26 of the respectivecylinders via a driver 134.

The CPU core 130 also drives, via a driver 136, through theelectromagnetic valve (TRPV) 68 and the valve actuator 64 (not shown inFIG. 6) to operate the switch-over valve 60, and the CPU core 130further discriminates whether the adsorbent 74 has degraded ordeteriorated. Here, the fact that “the adsorbent 74 has degraded ordeteriorated” indicates the HC adsorption or ability of the adsorbent 74has degraded or deteriorated

Furthermore, the CPU core 130 drives the EACV 24 and the EGR controlvalve 84 through drivers 138, 140. Moreover, the CPU core 130 lights awarning lamp 144 (not shown in FIG. 1) through a driver 142.

Before entering into the operation of the degradation discriminationsystem of an internal combustion engine exhaust gas purification systemaccording to the embodiment, the operation of the exhaust gaspurification system using the adsorbent 74 will be described withreference to FIG. 7.

In the exhaust gas purification system illustrated in the figure, whenthe engine 10 is cold-started, the switch-over valve 60 is kept in theposition shown by the solid lines in FIG. 2 for a predetermined periodof time (e.g., 40 sec.) since engine starting such that the main exhaustgas passage 38 a is closed, while the bypass exhaust gas passage 56 isopened, as illustrated in FIG. 7A.

Since the first and second catalysts 40, 42 provided upstream of theadsorbent 74 have not been activated during the aforesaid period of timewhen the engine 10 was cold-started, the exhaust gas is not purified bythe catalysts 40, 42. The exhaust gas flows through the bypass exhaustgas passage 56, as shown by arrows in FIG. 7A, and the unburned HCcomponent is adsorbed by the adsorbent 74.

As the upstream catalysts 40, 42 are normally activated after a lapse ofthe predetermined period of time, the switch-over valve 60 is driven tothe position shown by phantom lines in FIG. 2 such that the main exhaustgas passage 38 a is opened, while the bypass exhaust gas passage 56 isclosed, as illustrated in FIG. 7B.

Accordingly, the exhaust gas purified by the upstream catalysts 40, 42flows through the main exhaust gas passage 38 a and heats the adsorbent74. As a result, the unburned HC component adsorbed by the adsorbent 74begins to desorb. Since the pressure of the exhaust gas flowing throughthe main exhaust gas passage 38 a is greater than that flowing throughthe bypass exhaust gas passage 56, a part or portion of the exhaust gasenters the bypass exhaust gas passage 56 through the holes 76.

Then as illustrated in FIG. 7C, the desorbed unburned HC component isrecirculated back to a position upstream of the first and secondcatalysts 40, 42, more specifically to the engine intake system throughthe EGR conduit 82, when the EGR is conducted. At this time, a part orportion of the exhaust gas flowing through the main exhaust gas passage38 a is sucked in by the negative pressure generated at the engineintake system and enters the bypass exhaust gas passage 56 through theholes 76.

The introduced exhaust gas flows through the bypass exhaust gas passage56 in a direction opposite to that of the exhaust gas flowing throughthe main exhaust gas passage, while accelerating or expediting theheating of the adsorbent 74, and is recirculated into the engine intakesystem to be burned once again. The exhaust gas generated by thisre-burning is purified by the upstream catalysts 40, 42 and flows out ofthe engine 10 through the main exhaust gas passage 38 a.

Based on the above, the mode of operation of the degradationdiscrimination system of an internal combustion engine exhaust gaspurification system according to the embodiment of the invention will beexplained.

Outlining the operation with reference to FIGS. 7 and 8, in theoperation of the system, more precisely in the discrimination ofdegradation of the adsorbent 74, when the HC concentration is measuredat a point a and at a point β (the point where the HC sensor 104 islocated) as shown in FIG. 7A, the measured result indicative of thebehavior of the HC will be as shown in FIG. 8. From the measured resultillustrated in FIG. 8, it can be found that, due to the adsorptioneffect of the adsorber 74, the HC concentration at the downstream pointis lower than that of exhaust gas flowing into the adsorbent 74.

The invention was made based on this fact and the system according tothis invention is configured to detect the HC concentration (hereinafterreferred to as “trs.hc”) at the rear end of the adsorbent 74 (or at alocation downstream of the adsorbent) through the HC sensor 104, and tocompare the detected HC concentration with a threshold value (fordegradation discrimination, hereinafter referred to as “trs.hc.agd”)such that the degradation can be discriminated based on the result ofcomparison as illustrated in FIGS. 9 and 10. The system is configured todetermine the threshold value trs.hc.agd to be a value appropraitelyindicative of the HC concentration to be compared with the detectedvalue.

More specifically, in the case shown in FIG. 9, the threshold valuetrs.hc.agd is greater or equal to the detected HC concentration trs.hc.Since this indicates that the HC concentration trs.hc at the downstreampoint is less than or lower than the HC concentration in the exhaust gasflowing into the adsorbent (this concentration being hereinafterreferred to as “ex.hc” ), it can be discriminated that the adsorbent hasnot degraded or deteriorated. On the other hand, in the case shown inFIG. 10, since the detected HC concentration trs.hc is greater than thethreshold value trs.hc.agd, it can be discriminated that the adsorbenthas degraded or deteriorated.

The above will be explained in detail with reference to the flow chartof FIG. 11. The program in FIG. 11 is executed when the ignition switch(not shown) is made on and is looped once every 100 msec.

The program begins at S10 in which temperatures are estimated ordetected.

FIGS. 12 and 13 are first and second halves of flow charts showing thesubroutine for this.

The program begins in S100 in which a basic value of a fore-catalystexhaust gas temperature (named “tmphat.exbase” ) is estimated ordetermined. The fore-aft exhaust gas temperature is an estimatedtemperature of the exhaust gas flowing into the first and secondcatalysts 40, 42 (i.e. the gas immediately exhausted from the combustionchamber 28) and the value tmphat.exbase is a base value thereof.

Specifically, this is done by retrieving mapped data (whosecharacteristics are shown in FIG. 14) using the detected engine load(e.g. manifold absolute pressure PBA), the detected engine speed NE, theair/fuel ratio being operated, the ignition timing, etc. as addressdata.

More specifically, tmphat.exbase is retrieved using a characteristicshown by a solid line in the figure when the detected engine speed NE iswithin a range. On the other hand, tmphat.exbase is retrieved usingupper characteristics shown by phantom lines when the detected enginespeed NE is out of the range in the higher direction, or when theair/fuel ratio being operated is at the stoichiometric air/fuel ratio orthereabout, or when the ignition timing is retarded. Otherwisetmphat.exbase is retrieved using a lower characteristics shown byphantom lines.

It should be noted here that, if the engine 10 was a direct injectionspark ignition engine in which gasoline is directly injected into thecombustion chamber, the fuel injected in the intake stroke generates apre-mixture combustion, while that in the compression stroke generates astratified combustion. Thus, since the form of combustion is differentwith the fuel injection timing in the direct injection spark ignitionengine, it would alternatively be possible to change the characteristicsby the form of combustion when the engine 10 was a direct injectionspark ignition engine.

The program then proceeds to S102 in which a post-engine-startingexhaust gas temperature correction coefficient (named “k.tmpex”) isdetermined or calculated. This is done by retrieving mapped data (whosecharacteristics are shown in FIG. 15) using a value of apost-engine-starting timer (upcounter; the timer value is hereinafterreferred to as “tm.trs”) indicative of a period of time since enginestarting and the detected engine coolant temperature TW as address data.

Specifically, k.tmpex is retrieved using a characteristic shown by asolid line in the figure when the detected engine coolant temperature TWis within a range. On the other hand, k.tmpex is retrieved using lowercharacteristics shown by phantom lines when the detected engine coolanttemperature TW is out of the range in the lower direction, while k.tmpexis retrieved using upper characteristics shown by phantom lines when thedetected engine coolant temperature TW is out of the range in the higherdirection.

In the characteristics shown in FIG. 15, the reason why k.tmpex is setto be smaller at a low TW than that at a high TW, is that, since theengine temperature is low at a low engine coolant temperature TW suchthat the heat generated by the exhaust gas is immediately emitted fromthe engine 10, the exhaust gas temperature drops accordingly.

The program then proceeds to S104 in which, as shown there, thefore-catalyst exhaust gas temperature tmphat.ex is determined(estimated) by multiplying the basic value tmphat.exbase by thecorrection coefficient k.tmpex.

The program then proceeds to S106 in which it is determined whether thesystem has a catalyst temperature sensor. A system according to a secondembodiment has the sensor, however, the system according to thisembodiment is not configured to have the sensor, the result in S106 isnegative and the program proceeds to S108 in which the catalysttemperature is estimated or determined using a dynamic model describedby an equation shown there. The estimated catalyst temperature ishereinafter referred to as “tmphat.cat”.

In the equation, the suffix “n” represents a sampling number in thediscrete-time system, specifically, the time at which the program ofFIG. 11 flow chart is executed, more specifically, (n) indicates thecurrent program-execution-time and (n-n) indicates theprogram-execution-time m-time earlier. For brevity, addition of (n) tovalues at the current time is often omitted.

Further, in the equation, “d” indicates a dead time. And, values “at1”,“at2” and “bt1” indicate coefficients and are determined as valuesranging from −1 to +2 by retrieving data from a table (whosecharacteristics are shown in FIG. 16) using the estimated catalysttemperature tmphat.cat, more precisely its last valve tmphat.cat(n−1) asaddress data.

The catalyst temperature tmphat.cat is thus estimated or calculatedusing a dynamic model constituted as a DARX model (auto-regressive modelhaving a dead time in its input), in view of the dynamics of thecatalysts 40, 42 including heat adsorption, the degree of activation andtemperature change.

The program then proceeds to S110 in which the estimated catalysttemperature tmphat.cat is rewritten as the catalyst temperature tmp.cat.In other words, the estimated value is used as a substitution for theactual value.

The program then proceeds to S112 in which it is determined whether thesystem has an aft-catalyst temperature sensor. A system according to athird embodiment has the sensor, however, the system according to thisembodiment is not configured to have the sensor, the result in S112 isnegative and the program proceeds to S114 in which the aft-catalystexhaust gas temperature (hereinafter referred as to “tmphat.acat”) isestimated or determined. This is done by retrieving mapped data (whosecharacteristics are shown in FIG. 17) using the catalyst temperaturetmp.cat and the estimated fore-catalyst exhaust gas temperaturetmphat.ex as address data.

More specifically, tmphat.acat is retrieved using a characteristic shownby a solid line in FIG. 17 when the estimated fore-catalyst exhaust gastemperature tmphat.ex is within a range. On the other hand, when theestimated temperature tmphat.ex is out of the range, tmphat.acat isretrieved using an upper characteristic or a lower characteristic shownby phantom lines depending on the direction in which the estimatedtemperature tmphat.ex is out of the range. Similar to the determinationof the value tmphat.exbase, it would alternatively be possible to changethe characteristics shown in FIG. 17 by the form of combustion when theengine 10 was a direct injection spark ignition engine.

In the flow chart of FIG. 12, the program proceeds to S116 in which theestimated aft-catalyst exhaust gas temperature tmphat.acat is rewrittenas the aft-catalyst exhaust gas temperature (named “tmp.acat”). In otherwords, the estimated value is used as a substitution for the actualvalue.

The program then proceeds to S118 in which it is determined whether thesystem is provided with an adsorbent temperature sensor. A systemaccording to a fourth embodiment has the sensor, however, the systemaccording to this embodiment is not configured to have the sensor, theresult in S118 is negative and the program proceeds to S120 (in the flowchart of FIG. 13) in which it is determined whether the bit of a flagh.hctrs.on is set to 1. To set the bit of the flag to 1 indicates toissue the instruction to operate the switch-over valve 60 such that thebypass exhaust gas passage 56 is opened, while to reset it to 0indicates to generate the instruction to operate the valve 60 such thatthe bypass exhaust gas passage 56 is closed.

Since the instruction is generated to operate the switch-over valve 60to open the bypass exhaust gas passage 56 for a predetermined period oftime (e.g. 40 sec.), since engine starting, the result in S120 isnormally affirmative in the first program loop and the program proceedsto S122 in which it is determined whether the bit of a flag f.hc.pg(initially reset to 0) is set to 1. As mentioned above, the bypassexhaust gas passage 56 is opened for the predetermined period of time(the adsorption mode). However, if the adsorbed HC substantially beginsto desorb before the expiration of the predetermined period of time, thebit of the flag is set to 1, indicating that the instruction to closethe bypass exhaust gas passage 56 should be issued.

Accordingly, the judgement in S122 corresponds to determine whether thedesorption of HC has begun. When the result is negative, since thisindicates that it is still in the adsorption mode, the program proceedsto S124 in which it is determined whether the adsorbent temperature isestimated to be a predetermined temperature X.TMP.TRAP (approximately inbetween 50° C. to 60° C., for example). This is because the adsorbenttemperature tmp.trs is maintained at a certain temperature (i.e. 50° C.to 60° C.) in the adsorption mode due to the influence of the heat ofvaporization of moisture adsorbed together with HC. This estimatedadsorbent temperature is hereinafter referred to as “tmphat.trs”.

The program then proceeds to S126 in which the estimated adsorbenttemperature tmphat.trs is rewritten as the adsorbent temperature (named“tmp.trs”). In other words, the estimated value is used as asubstitution for the actual value.

On the other hand, when the result in S122 is affirmative, since thisindicates the desorption mode has begun, the program proceeds to S128 inwhich the estimated adsorbent temperature tmphat.trs is determined usinga dynamic model described by an equation shown there. This determinationis the same as the estimated catalyst temperature tmphat.cat. In theequation, values “at1t”,“at2t” and “bt1t” indicate coefficients and aredetermined as values ranging from −1 to +2 by retrieving data from atable (whose characteristics are shown in FIG. 18) using the estimatedadsorbent temperature tmphat.trs, more precisely its last valvetmphat.trs (n−1) as address data.

When the result in S120 is negative, since this indicates that thebypass exhaust gas passage 56 is kept closed, the program proceeds toS130 in which the estimated adsorbent temperature tmphat.trs isdetermined or calculated in a similar manner. FIG. 19 is a graph showingthe characteristics of coefficients “at1p”, “at2p” and “bt1p” used inthe equation illustrated in S130.

Returning to the explanation of the flow chart of FIG. 11, the programproceeds to S12 in which the temperature ex.hc indicative of anaft-catalyst in-exhaust gas HC concentration is estimated. The estimatedaft-catalyst in-exhaust gas HC concentration indicates the HCconcentration of the exhaust gas currently generated by the engine 10 inthe exhaust gas to be recirculated. In other words, this value ex.hcindicates the HC concentration (other than that of the desorbed HC) inthe exhaust gas flowing downstream through the first and secondcatalysts 40, 42.

FIG. 20 is a flow chart showing the subroutine of this.

The program begins in S200 in which a basic value of the aft-catalystin-exhaust gas HC concentration (named “ex.hc.base”) is calculated orestimated. This is done by retrieving mapped data (whose characteristicsare shown in FIG. 21) using the detected engine load (manifold absolutepressure PBA) and the engine speed NE as address data. Specifically, itis retrieved using a characteristic shown by a solid line in FIG. 21when the engine speed NE is within a range. If not, it is retrievedusing an upper characteristic or a lower characteristic shown by phantomlines depending on the direction in which the engine speed NE is out ofthe range.

The program then proceeds to S202 in which an engine-warmup correctioncoefficient k.engtmp is calculated. This is done by retrieving mappeddata (whose characteristics are shown in FIG. 22) using the time sinceengine starting (the timer value tm.trs) and the detected engine coolanttemperature TW. Specifically, it is retrieved using a characteristicshown by a solid line in FIG. 22 when the engine coolant temperature TWis within a range. If not, it is retrieved using an upper characteristicor a lower characteristic shown by phantom lines depending on thedirection in which the engine coolant temperature TW is out of therange.

The program then proceeds to S204 in which a catalyst purification ratek.itacat is calculated. If the value k.itacat is 1, that indicates thatthe purification rate is 100%. This is done by retrieving mapped data(whose characteristics are shown in FIG. 23) using the catalysttemperature tmp.cat, the detected engine speed NE and the engine load(e.g. manifold absolute pressure PBA). Since the catalyst purificationrate k.itacat decreases with increasing engine speed NE and the manifoldabsolute pressure PBA, the characteristics of the rate are set as shownin the figure. Specifically, it is retrieved using a characteristicshown by a solid line in the figure when the engine speed NE and themanifold absolute pressure PBA are within a range. If not, it isretrieved using an upper characteristic or a lower characteristic shownby phantom lines depending on the direction in which the engine speed NEand the manifold absolute pressure PBA are out of the range.

The program then proceeds to S206 in which the basic value of theaft-catalyst in-exhaust gas HC concentration ex.hc.base is multiplied bythe engine-warmup correction coefficient k.engtmp and the catalystpurification rate k.itacat to determine the product as the estimatedaft-catalyst in-exhaust gas HC concentration ex.hc.

Again returning to the explanation of the flow chart of FIG. 11, theprogram proceeds to S14 in which the HC concentration trs.hc isdetermined or calculated.

FIG. 24 is a flow chart showing the subroutine for this determination.

The program begins in S300 in which it is determined whether the HCsensor 104 is active. Since the HC sensor 104 has not been activateduntil the ambient temperature rises to a predetermined value, this isdone by measuring time lapse since engine starting and when the measuredtime has not reached a prescribed time, it is determined that the HCsensor 104 is not active.

When the result in S300 is negative, the program proceeds to S302 inwhich the HC concentration trs.hc is estimated (calculated) byretrieving mapped data (whose characteristics are shown in FIG. 25)using the measured time since engine starting (i.e. the timer valuetm.trs) and the detected engine coolant temperature TW as address data.

To be more specific, the HC concentration trs.hc is retrieved using acharacteristic shown by a solid line in the figure when the detectedengine coolant temperature TW is within a range. On the other hand,trs.hc is retrieved using upper characteristics shown by phantom lineswhen the detected engine coolant temperature TW is out of the range inthe lower direction, while trs.hc is retrieved using lowercharacteristics shown by phantom lines when the detected engine coolanttemperature TW is out of the range in the higher direction.

In the characteristics shown in FIG. 25, the reason why trs.hc is set tobe larger at a low TW than that at a high TW, is that, the amount of HC(i.e. HC concentration) in the exhaust gas increases when the engine 10is cold. Thus, since the substitute value is calculated in S302 when theresult in S300 is negative, the influence of inactiveness of the HCsensor 104 can be reduced as least as possible. On the other hand, whenthe result in S300 is affirmative, the program proceeds to S304 in whichthe HC concentration trs.hc is simply determined by reading the outputof the HC sensor 104.

Returning to the explanation of FIG. 11, the program proceeds to S16 inwhich the times (named “tm.trs.ch1, 2”) defining a degradationdiscrimination period are determined. As illustrated in FIG. 9,tm.trs.ch1 indicates the time at which the degradation discriminationshould be initiated and tm.trs.ch2 indicates the time at which thedegradation discrimination should be terminated.

The determination of tm.trs.ch1, 2 is done by retrieving mapped data(whose characteristics are shown in FIG. 26) using an estimated adsorbedHC amount (named “hcm.hat”) and one from among the parameters relatingto the adsorbent temperature. It should be noted that the valuestm.trs.ch1, 2 are determined such that the degradation discriminationperiod is present after the HC sensor 104 has been activated.

The value hcm.hat is the amount of HC estimated to be adsorbed in theadsorbent 74. In the first program loop, the last value stored in abackup portion of the RAM 124 before the engine 10 was stopped is usedin the first program loop of the flow chart of FIG. 11.

As the adsorbent temperature parameter, the adsorbent temperaturetmp.trs should preferably be used. However, it is alternatively possibleto use another value such as the engine coolant temperature TW, thecatalyst temperature tmp.cat, the aft-catalyst exhaust gas temperaturetmp.acat, etc. As will be explained later, the estimated adsorbed HCamount hcm.hat is decreased in response to the desorption (purging) ofthe adsorbed HC and set to zero when the desorption (purging) has beencompleted.

In FIG. 26, a value X.TM.TRSLMT in the ordinate indicative of time axisdefined by the time since engine starting (the timer value tm.trs) meansthe aforesaid predetermined period of time defining the adsorption mode.

It should be noted, that a value or parameter having the prefix “X” inthis specification and corresponding figures indicates a predeterminedvalue or parameter.

In the flow chart of FIG. 11, the program proceeds to S18 in which it isdetermined whether the engine 10 has started. This is done bydetermining whether the engine 10 has started cranking and the fuelinjection has been started. If the cranking has started, but the fuelinjection has not, it is determined that the engine 10 has not started.

The result in S18 is normally negative in the first program loop and theprogram proceeds to S20 in which the timer value tm.trs is reset tozero. The program then proceeds to S22 in which it is determined whetherthe bit of a flag f.hctrs.on is set to 1. To set the bit of the flag to1 indicates to issue the instruction to operate the switch-over valve 60such that the bypass exhaust gas passage 56 is opened, while to reset itto 0 indicates to generate the instruction to operate the valve 60 suchthat the bypass exhaust gas passage 56 is closed. In S22, theinstruction is generated to operate the switch-over valve 60 to open thebypass exhaust gas passage 56 to begin the HC adsorption and enters theadsorption mode and the program is once terminated. The operation of theswitch-over valve 60 itself is conducted by the ECU 114 through thevalve actuator 64 in a routine (not shown).

When the result in S18 is affirmative in the next or later program loop,the program proceeds to S24 in which it is determined whether the flagf.hctrs.on is set to 1, more precisely it is in the adsorption mode. Theresult is normally affirmative in the first program loop, the programproceeds to S26 in which adsorbed HC amount hcm.hat is estimated ordetermined.

FIG. 27 is a flow chart showing the subroutine for this estimation.

The program begins in S400 in which the exhaust gas volume (named“trs.sv”) in terms of a space velocity through the adsorbent, isestimated or determined using an equation illustrated there. Theequation is an approximation using a value X.SVPRA. The value X.SVPRAis, for example, 65.74 assuming that the displacement volume of theengine 10 is 2.2 liters. The exhaust gas volume trs.sv may alternativelybe estimated based on another equation using the engine speed NE and thefuel injection amount TI.

The program then proceeds to S402 in which an HC density correctioncoefficient k.hc is determined or calculated by retrieving table data(whose characteristic is shown in FIG. 28) using the estimatedaft-catalyst exhaust gas temperature tmphat.acat as address data.

The program then proceeds to S404 in which the estimated adsorbed HCamount hcm.hat is determined or calculated using an equation illustratedthere. As mentioned above, the estimated adsorbed HC amount hcm.hat(initially set to zero) indicates the total amount of HC estimated to beadsorbed in the adsorbent 74.

In the equation illustrated in S404, X.HCS.P1 is a correctioncoefficient to be varied with the position of the HC sensor 104. Whenthe HC sensor 104 is located at the position shown in FIG. 1, the valueX.HCS.P1 should be larger than 1. To be more specific, assuming X.HCS.P1is 1 when the HC sensor 104 is located at a point downstream (in termsof the exhaust gas stream in the adsorption mode) of the adsorbent 74,i.e. in the right in FIG. 1, the HC sensor 104 or 104 a that is locatedat a position in the upstream side is set to an appropriate value largerthan 1. The determined value hcm.hat is stored in the backup portion ofthe RAM 124 and is kept there after the engine 10 has been stopped.

Returning to the explanation of FIG. 11, the program proceeds to S28 inwhich the timer value tm.trs is incremented by a prescribed amount X.TM.TRSJUD. In other words, the measurement of time laps after enginestarting is started.

The program then proceeds to S30 in which it is determined whether thevalue of the timer tm.trs has reached the aforesaid value X.TM.TRSLMT.The result is normally negative in the first program loop and theprogram proceeds to S32 in which it is determined whether the detectedHC concentration trs.hc is greater than an appropriately set thresholdvalue X.HC.TRSLMT.

As mentioned above with reference to FIG. 8, the detected HCconcentration is relatively low in the adsorption mode since the flowingHC is adsorbed to the adsorbent 74, but becomes larger when thedesorption begins. From this, it can be determined whether thedesorption of HC substantially begins by comparing the detected HCconcentration trs.hc with the threshold value X.HC.TRSLMT and bydetermining if the detected value trs.hc exceeds the threshold valueX.HC.TRSLMT.

When the result in S32 is negative, since this indicates that desorptionhas not begun, the program proceeds to S34 in which the threshold value(for degradation discrimination) trs.hc.agd is determined or calculated(explained later), and to S36 in which it is determined whether thetimer value tm.trs is within the range (degradation discriminationperiod) defined by the times tm.trs.ch1, 2. When the result isaffirmative, the program proceeds to S38 in which it is discriminatedwhether the adsorbent 74 has degraded (explained later). On the otherhand, when the result is negative, the program skips S38 and proceeds toS22.

On the other hand, when the result in S32 is affirmative, the programproceeds to S40 in which the bit of the flag f.hc.pg is set to 1, and toS42 in which a desorption-beginning-determination timer (named“tm.trs.full(n)”) is incremented by a prescribed amount X.TM.TRSJUD, toS44 in which it is determined whether the value of the timer is greateror equal to a threshold value X.TMFULL.D. When the result is negative,the program proceeds to S22. When the result is affirmative, the programproceeds to S46 in which the bit of the flag f.hctrs.on is reset to 0such that the bypass exhaust gas passage 56 is closed. The same willalso be applied if the result in S30 is affirmative.

Thus, by comparing the detected HC concentration with the thresholdvalue, it can be determined if the adsorption mode has terminated and ifit does, the bypass exhaust gas passage 56 is closed, thereby enablingto prevent the desorbed HC from flowing out downstream. Moreover, byproviding a delay time X.TMFULL.D, the termination of the adsorptionmode can be determined more accurately.

In the flow chart of FIG. 11, the determination of the threshold valuetrs.hc.agd mentioned in S34 will be explained.

FIG. 29 is a flow chart showing the subroutine of this.

The program begins in S500 in which a basic value of the adsorption rateof the adsorbent 74 (named “ita.trs.base”) is determined or calculated.This is done by retrieving mapped data (whose characteristics are shownin FIG. 30) using the adsorbent temperature parameter and the estimatedadsorbed HC amount hcm.hat as address data. Specifically, this is doneby retrieving a characteristic shown by a solid line in FIG. 30 when theestimated adsorbed HC amount hcm.hat is zero. If hcm.hat is larger thanzero, it is retrieved using one of the characteristics shown by phantomlines depending on the magnitude.

Since the adsorption rate basic value ita.trs.base varies with theadsorbent temperature, the value is retrieved using the adsorbenttemperature parameter. As the adsorbent temperature parameter, theadsorbent temperature tmp.trs is used. However, it is alternativelypossible to use another value such as the engine coolant temperature TW,the catalyst temperature tmp.cat, the aft-catalyst exhaust gastemperature tmp.acat, etc.

The program then proceeds to S502 in which an adsorber-adsorbing-ratecorrection coefficient kita.trs is calculated. This is done byretrieving table data (whose characteristics are shown in FIG. 31) usingthe time since engine starting (the timer value tm.trs) as address data.

The program then proceeds to S504 in which the calculated basic valueita.trs.base is multiplied by the correction coefficient kita.trs todetermine an adsorbent adsorption rate ita.trs. The rate indicates theadsorption rate of the adsorbent 74 and if it is 1, that indicates thatthe adsorption rate is 100%.

The program then proceeds to S506 in which the difference between 1 andthe calculated rate is calculated and the difference is multiplied bythe estimated in-exhaust gas HC concentration ex.hc to determine a valuehchat.trs. This value hchat.trs indicates an estimated HC concentrationat the rear end of the adsorber 74 or at a position downstream of theadsorbent 74.

The program then proceeds to S508 in which a predetermined valueX.TRS.AGD (positive value) is added to the estimated aft-adsorber HCconcentration hchat.trs and the sum is determined as the threshold value(for degradation discrimination) trs.hc.agd. In order to improve thediscrimination accuracy, the estimated aft-adsorbent HC concentration isnot immediately used. Instead, the predetermined value X.TRS.AGD isadded thereto. The predetermined value X.TRS.AGD may be varied with thetime since engine starting (timer value tm.trs) or the adsorbenttemperature parameter such as the adsorbent temperature tmp.trs.

In the flow chart of FIG. 11, the discrimination of adsorbentdegradation mentioned in S38 will be explained. As will be understoodfrom the figure, this discrimination is conducted when it is within thedegradation discrimination period in the adsorption mode.

FIG. 32 is a flow chart showing the subroutine for this discrimination.

The program begins in S600 in which it is determined whether the bit ofa discrimination-execution flag (named “ftrs.agd.ch”) is set to 1. Sincethe bit of the flag is initially reset to 0 and is set to 1 when theadsorbent 74 has been discriminated to ge degradeed, the procedure inthis step amounts for determining whether the adsorbent 74 has beendiscriminated to be degraded.

When the result in S600 is affirmative, the program is immediatelyterminated. When the result is negative, the program proceeds to S602 inwhich it is determined whether the detected HC concentration trs.hc isgreater than or exceeds the threshold value trs.hc.agd.

When the result in S602 is negative, since it can be determined that theadsorbent 74 has not degraded or deteriorated, the program proceeds toS604 in which the bit of the flag f.trs.agd is reset to 0.

On the other hand, when the result in S602 is affirmative, since it canbe determined that the ability or capacity of the adsorbent 74 degradesor deteriorates and hence, the adsorbent 74 has degraded or deterioratedas mentioned with reference to FIG. 10, the program proceeds to S606 inwhich the bit of a flag f.trs.agd is set to 1 and the warning lamp 144lit to inform the result to the operator.

The program then proceeds to S608 in which the bit of the flagf.trs.agd.ch is set to 1. With this, the result in S600 in the next orlater program loop is affirmative and the program is immediatelyterminated. Thus, since the adsorbent 74 was once discriminatated to bederaded, the discrimination will no longer be repeated in this vehicletrip.

Returning to the explanation of the flow chart of FIG. 11, when theresult in S24 is negative, the program proceeds to S48 in which it isdiscriminated whether the purging (recirculating) of desorbed HC hasbeen completed.

FIG. 33 is a flow chart showing the subroutine for this discrimination.

The program begins in S700 in which it is determined whether the bit ofa purging-completion-determination flag f.trs.purge is set to 1 and ifthe result is affirmative, the program is immediately terminated. Sincethe bit of the flag is set to 1 when the purging (recirculating) of thedesorbed HC has been completed, the result is normally negative in thefirst program loop and the program proceeds to S702 in which it isdetermined whether the EGR (Exhaust Gas Recirculation) operation is inprogress.

The EGR is determined to be operative or inoperative, in anotherroutine, based on the main engine parameters (defined by the enginespeed NE and the engine load (manifold absolute pressure PBA) and theengine warmup condition (determined from the engine coolant temperatureTW)), in view of other engine operating conditions (such as whether theengine 10 is under idling or the wide-open-throttle enrichment or thesupply of fuel is cut off, etc).

When the result in S702 is negative, since the purging is only conductedwhen the EGR operation is in progress, the program proceeds to S704 inwhich the estimated adsorbed HC amount hcm.hat is held or maintained. Onthe other hand, when the result in S702 is affirmative, since it can bedetermined that the purging is in progress, the program proceeds to S706in which it is determined whether the adsorbent temperature tmp.trs isgreater than or exceeds a threshold value X. TMP.PRG to determinewhether the desorption (purging) has substantially begun for the reasonmentioned above. The threshold value X.TMP.PRG is set to a temperatureranging from 250° C. to 400° C., for example, for the adsorbent 74 usedin this embodiment.

When the result in S706 is negative, since this indicates that thepurging has not begun, the program proceeds to S704. On the other hand,when the result in S706 is affirmative, the program proceeds to S708 inwhich it is determined whether the detected HC concentration trs.hc isgreater than the estimated in-exhaust gas HC concentration ex.hc. Whenthe result is negative, the program proceeds to S710 in which a countervalue (named “cnt.trs.prg”), more precisely its last time valuecnt.trs.prg(n−1) is incremented to be updated, to S712 in which it isdetermined whether the current counter value cnt.trs.prg(n) is greateror equal to a threshold value for discriminating purge completion (named“X.TRS.PRG”).

When the result in S712 is affirmative, since it can be determined thatthe purging has been completed (the desorbed HC has been whollyrecirculated), the program proceeds to S714 in which the estimatedadsorbed HC amount hcm.hat is set to zero. With this, it can preventestimation errors from being accumulated. The program then proceeds toS716 in which the purge-completion-determination flag f.trs.purge is setto 1.

Explaining this, the fact that the detected HC concentration trs.hc isless than or equal to the HC concentration of exhaust gas newly flowing,would indicate the purging has terminated. In order to make sure,however, it is, as a precaution, checked whether this conditioncontinues for a predetermined period of time (in program loops, i.e.X.TRS.PRG) and when it does, it is determined that the purging has beencompleted. With this, it becomes possible to determine the completion ofpurging.

On the other hand, when the result in S708 is affirmative, since thisindicates that the purging has not been completed, the program proceedsto S718 in which the counter value is reset to zero. This will be thesame when the counter value is once incremented in S710, but is negativein this step in the next or later program loop.

When the result is S712 is negative, the program proceeds to S720 inwhich the EGR flow rate q.egr is estimated or determined. The valueq.egr indicates the total amount of recirculated exhaust gas includingthe desorbed HC. The value ex.hc is determined by retrieving table data(whose characteristics are shown in FIG. 34) using the detected EGRcontrol valve lift amount lact as address data. Specifically, it isretrieved using a characteristic shown by a solid line in FIG. 34.

It is alternatively possible to configure such that the value q.egr isretrieved using the characteristic illustrated by the solid line whenthe engine load (manifold absolute pressure PBA) is within a range andif not, it is retrieved using an upper characteristic or a lowercharacteristic shown by phantom lines depending on the direction inwhich the engine load (manifold absolute pressure PBA) is out of therange. Further, instead of the detected EGR control valve lift amountlact, a command value thereto may be used.

The program then proceeds to S722 in which the HC density correctioncoefficient (during EGR) k.hc.egr is determined or calculated. This isdone by retrieving table data (whose characteristic is shown in FIG. 35)using the estimated adsorbent temperature tmp.trs as address data.

The program then proceeds to S724 in which the estimated adsorbed HCamount hcm.hat is again determined or corrected using an equationillustrated there. The determined value is similarly stored in thebackup portion of the RAM 124 to be used in S16 in the flow chart ofFIG. 11 if desired.

In the equation illustrated in S724, X.HCS.P2 is a correctioncoefficient similar to that of X.HCS.P1 mentioned with reference to S404in the flow chart of FIG. 27. Here, the value X.HCS.P2 should be suchthat it reaches 1 as the location of HC sensor is at a more downstreamposition (in terms of the exhaust gas stream in the desorption mode; inthe left in FIG. 1). In other words, X.HCS.P2 should be decreased as thesensor location is moved to the entrance of the EGR conduit 82.

The program then proceeds to S726 in which the bit of thepurge-completion-determination flag f.trs.purge is reset to 0.

Since the degradation discrimination system for an internal combustionengine exhaust gas purification system according to this embodiment isconfigured to monitor the behavior of the HC through the HC sensor 104and to determine the HC concentration trs.hc in the adsorption mode suchthat it should be compared with the threshold value trs.hc.agd todiscriminate whether the adsorbent 74 has degraded or deteriorated, thesystem can discriminate the occurrence of adsorbent degradation withaccuracy. Since the system needs no calculation of HC amount (mass), thesystem can prevent any calculation error from occurring.

Further, since the system is configured to determine the threshold valuetrs.hc.agd using the adsorbent temperature parameter such as theadsorbent temperature tmp.trs, the system can determine the thresholdvalue accurately, thereby enabling to improve the degradationdiscrimination accuracy.

Furthermore, since the system is configured to determine the thresholdvalue trs.hc.agd using the estimated adsorbed HC amount, the system candetermine the threshold value accurately, thereby enabling to improvethe degradation discrimination accuracy. Since the threshold value iscorrected by the time since engine starting (tm.trs), this makes itpossible to describe the change in the adsorption rate of adsorbent withrespect to time, thereby enhancing the degradation discriminationaccuracy.

Furthermore, since the system is configured to determine the thresholdvalue trs.hc.agd using the estimated aft-catalyst HC concentration, thesystem can determine the threshold value accurately, thereby enabling toimprove the degradation discrimination accuracy.

Furthermore, since the system is configured to conduct the degradationdiscrimination in the period defined by tm.trs.ch1,2 and to determinethe period based on the parameter relating to the temperature of theadsorbent 74 and the estimated adsorbed HC amount hcm.hat, the systemcan conduct the degradation at a time suitable for that after the HCsensor 104 has been activated, thereby enabling to improve thedegradation discrimination accuracy.

Furthermore, since the system is configured to determine the estimatedadsorbed HC amount hcm.hat using the difference between the HCconcentration trs.hc and the estimated in-exhaust gas HC concentrationex.hc as disclosed in S404 in the flow chart of FIG. 27 (and in S724 inthe flow chart of FIG. 33), the system can determine the estimatedadsorbed HC amount accurately, thereby enabling to improve thedegradation discrimination accuracy.

Furthermore, since the system is configured to use a substitution valueas the detected HC concentration determined based on the engine coolanttemperature TW and the time since engine starting (tm.trs), the systemcan determine the estimated adsorbed HC amount accurately, therebyenabling to improve the degradation discrimination accuracy.

FIG. 36 is a cross sectional view of the portion downstream of the firstand second catalysts 40, 42 showing a partial configuration of adegradation discrimination system of an internal combustion engineexhaust gas purification system according to a second embodiment of theinvention.

In the second embodiment, as illustrated in the figure, in addition tothe HC sensor 104, a temperature sensor (catalyst temperature sensor)200 is installed at the first catalyst bed 42 a of the second catalyst42. The temperature sensor 200 generates a signal indicative of thetemperature of the first catalyst bed 42 a.

It is alternatively possible to install the temperature sensor 200 at aposition between the first catalyst bed 42 a and the second catalyst bed42 b as shown by phantom lines in the figure with the reference numeral200 a or at the second catalyst bed 42 b as shown by phantom lines withthe reference numeral 200 b.

Explaining this with focus on the differences from the first embodiment,in the determination of the estimated temperatures in a flow chart (notshown) to FIGS. 12 and 13, the result in a step similar to S106 isaffirmative and the program proceeds to a step similar to S132 in whichit is determined whether the catalyst temperature sensor 200 is active.

When the result is negative, the program proceeds to a step similar toS108 in the same manner in the first embodiment. On the other hand, whenthe result is affirmative, the program proceeds to a step similar toS134 in which the output indicative of tmp.cat of the catalysttemperature sensor 200 is read.

In the second embodiment, thus, when the catalyst temperature sensor 200has been activated, the sensor output is immediately read as thecatalyst temperature tmp.cat, which is used, instead of the estimatedvalue, in steps including that similar to S114 of the flow chart.

Having been configured in such a manner, the degradation discriminationsystem for an internal combustion engine exhaust gas purification systemaccording to the second embodiment can further enhance the degradationdiscrimination accuracy. The rest of the configuration as well as theeffects and advantages thereof is the same as the first embodiment.

FIG. 37 is a cross sectional view of the portion downstream of the firstand second catalysts 40, 42 showing a partial configuration of adegradation discrimination system of an internal combustion engineexhaust gas purification system according to a third embodiment of theinvention.

In the third embodiment, as illustrated in the figure, in addition tothe HC sensor 104, a temperature sensor (aft-catalyst temperaturesensor) 300 is installed at a position between the switch-over valve 60and the adsorbent 74, more precisely at a position upstream of theadsorbent (closer to the branch 52). The temperature sensor 300generates a signal indicative of the temperature at that locationdownstream of the catalyst 42.

It is alternatively possible to install the temperature sensor 300 at aposition between the second catalyst bed 42 b and the switch-over valve60 as shown by phantom lines in the figure with the reference numeral300 a.

Explaining this with focus on the differences from the first embodiment,in the determination of the estimated temperatures in a flow chart (notshown) to FIGS. 12 and 13, the result in a step similar to S112 isaffirmative and the program proceeds to a step similar to S136 in whichit is determined whether the aft-catalyst temperature sensor 300 isactive.

When the result is negative, the program proceeds to a step similar toS114 in the same manner in the first embodiment. On the other hand, whenthe result is affirmative, the program proceeds to a step similar toS138 in which the output indicative of tmp.acat of the catalysttemperature sensor 300 is read.

In the third embodiment, thus, when the aft-catalyst temperature sensor300 has been activated, the sensor output is immediately read as thecatalyst temperature tmp.acat, which is used, instead of the estimatedvalue, in steps including that similar to S402 in the flow chart of FIG.27.

Having been configured in such a manner, the degradation discriminationsystem for an internal combustion engine exhaust gas purification systemaccording to the third embodiment can further enhance the degradationdiscrimination accuracy. The rest of the configuration as well as theeffects and advantages thereof is the same as the first embodiment.

FIG. 38 is a cross sectional view of the portion downstream of the firstand second catalysts 40, 42 showing a partial configuration of adegradation discrimination system of an internal combustion engineexhaust gas purification system according to a fourth embodiment of theinvention.

In the fourth embodiment, as illustrated in the figure, in addition tothe HC sensor 104, a temperature sensor (adsorbent temperature sensor)400 is installed at the adsorbent 74, more precisely at the rear end(closer to the vehicle rear assembly 46) of the second adsorbent bed 74b. The temperature sensor 400 generates a signal indicative of thetemperature of the adsorbent 74.

It is alternatively possible to install the temperature sensor 400 at aposition between the first adsorbent bed 74 a and the second adsorbentbed 74 b as shown by phantom lines in the figure with the referencenumeral 400 a or at the first adsorbent bed 74 a as shown by phantomlines with the reference numeral 400 b.

Explaining this with focus on the differences from the first embodiment,in the determination of the estimated temperatures in a flow chart (notshown) to FIGS. 12 and 13, the result in a step similar to S118 isaffirmative and the program proceeds to a step similar to S140 in whichit is determined whether the adsorbent temperature sensor 400 is active.

When the result is negative, the program proceeds to a step similar toS120 in the same manner in the first embodiment. On the other hand, whenthe result is affirmative, the program proceeds to a step similar toS142 in which the output indicative of tmp.trs of the adsorbenttemperature sensor 400 is read.

In the fourth embodiment, thus, when the adsorbent temperature sensor400 has been activated, the sensor output is immediately read as thecatalyst temperature tmp.trs, which is used, instead of the estimatedvalue, in steps including that similar to S500 in the flow chart of FIG.29.

Having been configured in such a manner, the degradation discriminationsystem for an internal combustion engine exhaust gas purification systemaccording to the fourth embodiment can further enhance the degradationdiscrimination accuracy. The rest of the configuration as well as theeffects and advantages thereof is the same as the first embodiment.

The first to fourth embodiments are thus configured to have a system fordiscriminating degradation of an exhaust purification system of aninternal combustion engine (10) having an adsorbent (74) installed at abypass exhaust gas passage (56) branched from an exhaust pipe (8) of theengine and merged into the exhaust pipe at a location downstream of theadsorbent, the bypass exhaust gas passage being opened by a switch-overvalve (60) at starting of the engine to introduce the exhaust gas suchthat the adsorbent adsorbs unburned HC in exhaust gas generated by theengine and being closed such that the adsorbent desorbs the adsorbed HCand the desorbed HC is thereafter recirculated at a position upstream ofa catalyst (40, 42), comprising: an HC sensor (104, 104 a, 104 b, ECU114, S10) installed in the bypass exhaust gas passage at least one of afirst position at the adsorbent and a second position downstream of theadsorbent for detecting concentration of HC (trs.hc) introduced in thepass exhaust gas passage in an adsorption mode where the adsorbentadsorbs HC; and adsorbent degradation discriminating means (ECU 114,S38, S600 to S608) for comparing the detected concentration of HC with athreshold value (trs.hc.agd) and for discriminating whether theadsorbent has degraded based on a result of the comparison.

In the system, the adsorbent degradation discriminating means includes:engine operating condition detecting means (crank angle sensor 90,manifold absolute pressure sensor 94, coolant temperature sensor 96, ECU114) for detecting operating conditions of the engine; estimatedin-exhaust gas HC concentration determining means (ECU 114, S12, S200 toS206) for determining an estimated in-exhaust gas HC concentration(ex.hc) indicative of an estimated HC concentration in the exhaust gasbased at least on an engine speed and an engine load of the detectedengine operating conditions and the detected concentration of HC; andthreshold value determining means (ECU 114, S506 to S508) fordetermining the threshold value based at least on the determinedestimated in-exhaust gas HC concentration.

In the system, the adsorbent degradation discriminating means includes:exhaust gas volume determining means (ECU 114, S26, S400) fordetermining a volume of the exhaust gas (trs.sv) flowing in the bypassexhaust gas passage and into the adsorbent based at least on the enginespeed (NE) and the engine load (PBA) of the detected engine operatingconditions; estimated adsorbed HC amount determining means fordetermining an estimated amount of HC (hcm.hat) adsorbed by theadsorbent; and HC adsorption rate determining means (ECU 114, S500 toS504) for determining an HC adsorption rate (ita.trs) of the adsorbentbased at least on the determined estimated adsorbed HC amount; and thethreshold value determining means determines the threshold value basedat least on the estimated HC adsorption amount and the estimatedin-exhaust gas HC concentration.

In the system, the HC adsorption rate determining means determines theHC adsorption rate based on the estimated adsorbed HC amount (hcm.hat)and a parameter relating to a temperature of the adsorbent. Theparameter is at least one from among the temperature of the adsorbent(tmp.trs), a coolant temperature of the engine (TW), a temperature ofthe catalyst (tmp.cat) and a temperature at a location downstream of thecatalyst (tmp.acat). The parameter is an estimated parameter or ameasured parameter obtained by a temperature sensor (200, 300, 400).

In the system, the HC adsorption rate determining means includes:correction coefficient calculating means (ECU 114, S502) for calculatinga correction coefficient (kita.trs) based on a time since starting ofthe engine (tm.trs); and corrects the HC adsorption rate based on thedetermined correction coefficient.

In the system, the adsorbent degradation discriminating means includes:degradation discrimination period determining means (ECU 114, S16) fordetermining a period for degradation discrimination (tm.trs.ch1, 2)based at least on a parameter relating to a temperature of theadsorbent, more precisely, based at least on the parameter and theestimated adsorbed HC amount (hcm.hat); and in-period determining means(ECU 114, S36, S38) for determining whether it is within the period; anddiscriminates whether the adsorbent has degraded when it is within theperiod.

In the system, the threshold value determining means determines thethreshold value based at least on a parameter relating to a temperatureof the adsorbent (ECU 114, S34, S500 to S508).

In the system, the adsorbent degradation discriminating means lights awarning lamp (144) when the adsorbent is discriminated to be degraded(ECU 114, S606).

It should be noted in the above that, although the manifold absolutepressure PBA is used as the value indicative of the engine load, it isalternatively possible to use the air flow rate or the throttle opening.

It should also be noted that the switch-over valve may be opened orclosed by an electric actuator.

It should further be noted that the adsorbent should not be limited tothe type disclosed, any other type may be used if it has an excellentheat-proof property.

While the invention has thus been shown and described with reference tospecific embodiments, it should be noted that the invention is in no waylimited to the details of the described arrangements, changes andmodifications may be made without departing from the scope of theappended claims.

What is claimed is:
 1. A system for discriminating degradation of anexhaust purification system of an internal combustion engine having anadsorbent installed at a bypass exhaust gas passage branched from anexhaust pipe of the engine and merged into the exhaust pipe at alocation downstream of the adsorbent, the bypass exhaust gas passagebeing opened by a switch-over valve at starting of the engine tointroduce the exhaust gas such that the adsorbent adsorbs unburned HC inexhaust gas generated by the engine and being closed such that theadsorbent desorbs the adsorbed HC and the desorbed HC is thereafterrecirculated at a position upstream of a catalyst, comprising: an HCsensor installed in the bypass exhaust gas passage at least one of afirst position at the adsorbent and a second position downstream of theadsorbent for detecting concentration of HC introduced in the bypassexhaust gas passage in an adsorption mode where the adsorbent adsorbsHC; and adsorbent degradation discriminating means for comparing thedetected concentration of HC with a threshold value and fordiscriminating whether the adsorbent has degraded based on a result ofcomparison.
 2. A system according to claim 1, wherein the adsorbentdegradation discriminating means includes: engine operating conditiondetecting means for detecting operating conditions of the engine;estimated in-exhaust gas HC concentration determining means fordetermining an estimated in-exhaust gas HC concentration indicative ofan estimated HC concentration in the exhaust gas based at least on anengine speed and an engine load of the detected engine operatingconditions and the detected concentration of HC; and threshold valuedetermining means for determining the threshold value based at least onthe determined estimated in-exhaust gas HC concentration.
 3. A systemaccording to claim 2, wherein the adsorbent degradation discriminatingmeans includes: exhaust gas volume determining means for determining avolume of the exhaust gas flowing in the bypass exhaust gas passage andinto the adsorbent based at least on the engine speed and the engineload of the detected engine operating conditions; estimated adsorbed HCamount determining means for determining an estimated amount of HCadsorbed to the adsorbent; and HC adsorption rate determining means fordetermining an HC adsorption rate of the adsorbent based at least on thedetermined estimated adsorbed HC amount; and the threshold valuedetermining means determines the threshold value based at least on theestimated HC adsorption amount and the estimated in-exhaust gas HCconcentration.
 4. A system according to claim 3, wherein the HCadsorption rate determining means determines the HC adsorption ratebased on the estimated adsorbed HC amount and a parameter relating to atemperature of the adsorbent.
 5. A system according to claim 4, whereinthe parameter is at least one from among the temperature of theadsorbent, a coolant temperature of the engine, a temperature of thecatalyst and a temperature at a location downstream of the catalyst. 6.A system according to claim 5, wherein the parameter is an estimatedparameter.
 7. A system according to claim 5, wherein the parameter is ameasured parameter obtained by a temperature sensor.
 8. A systemaccording to claim 3, wherein the HC adsorption rate determining meansincludes: correction coefficient calculating means for calculating acorrection coefficient based on a time since starting of the engine; andcorrecting the HC adsorption rate based on the determined correctioncoefficient.
 9. A system according to claim 8, wherein the HC adsorptionrate determining means includes: correction coefficient calculatingmeans for calculating a correction coefficient based on a time sincestarting of the engine; and correcting the HC adsorption rate based onthe determined correction coefficient.
 10. A system according to claim2, wherein the adsorbent degradation discriminating means includes:degradation discrimination period determining means for determining aperiod for degradation discrimination based at least on a parameterrelating to a temperature of the adsorbent; and in-period determiningmeans for determining whether it is within the period; anddiscriminating whether the adsorbent has degraded when it is within theperiod.
 11. A system according to claim 10, wherein the parameter is atleast one from among the temperature of the adsorbent, a coolanttemperature of the engine, a temperature of the catalyst and atemperature at a location downstream of the catalyst.
 12. A systemaccording to claim 11, wherein the parameter is an estimated parameter.13. A system according to claim 11, wherein the parameter is a measuredparameter obtained by a temperature sensor.
 14. A system according toclaim 2, wherein the threshold value determining means determines thethreshold value based at least on a parameter relating to a temperatureof the adsorbent.
 15. A system according to claim 14, wherein theparameter is at least one from among the temperature of the adsorbent, acoolant temperature of the engine, a temperature of the catalyst and atemperature at a location downstream of the catalyst.
 16. A systemaccording to claim 15, wherein the parameter is an estimated parameter.17. A system according to claim 15, wherein the parameter is a measuredparameter obtained by a temperature sensor.
 18. A system according toclaim 1, wherein the adsorbent degradation discriminating means lights awarning lamp when the adsorbent is discriminated to be degraded.
 19. Amethod of discriminating degradation of an exhaust purification systemof an internal combustion engine having an adsorbent installed at abypass exhaust gas passage branched from an exhaust pipe of the engineand merged into the exhaust pipe at a location downstream of theadsorbent, the bypass exhaust gas passage being opened by a switch-overvalve at starting of the engine to introduce the exhaust gas such thatthe adsorbent adsorbs unburned HC in exhaust gas generated by the engineand being closed such that the adsorbent desorbs the absorbed HC and thedesorbed HC is thereafter recirculated at a position upstream of acatalyst, comprising the steps of: detecting concentration of HCintroduced in the bypass exhaust gas passage in an adsorption mode wherethe adsorpent adsorbs HC; comparing the detected concentration of HCwith a threshold value and for discriminating whether the adsorbent hasdegraded based on a result of the comparison; wherein the step ofadsorbent degradation discriminating includes the steps of: detectingoperating conditions of the engine; determining an estimated in-exhaustgas HC concentration indicative of an estimated HC concentration in theexhaust gas based at least on an engine speed and an engine load of thedetected engine operating conditions and the detected concentration ofHC; determining the threshold value based at least on the determinedestimated in-exhaust gas HC concentration; determining a volume of theexhaust gas flowing in the bypass exhaust gas passage and into theadsorbent based at least on the engine speed and the engine load of thedetected engine operating conditions; determining an estimated amount ofHC adsorbed to the adsorbent; and determining an HC adsorption rate ofthe adsorbent based at least on the determined estimated adsorbed HCamount; and the step of threshold value determining determines thethreshold value based at least on the estimated HC adsorption amount andthe estimated in-exhaust gas HC concentration.
 20. A method according toclaim 19, wherein the step of HC adsorption rate determining determinesthe HC adsorption rate based on the estimated adsorbed HC amount and aparameter relating to a temperature of the adsorbent.
 21. A methodaccording to claim 20, wherein the parameter is at least one from amongthe temperature of the adsorbent, a coolant temperature of the engine, atemperature of the catalyst and a temperature at a location downstreamof the catalyst.
 22. A method according to claim 21, wherein theparameter is an estimated parameter.
 23. A method according to claim 21,wherein the parameter is a measured parameter obtained by a temperaturesensor.
 24. A method according to claim 19, wherein the step of HCadsorption rate determining includes the step of: calculating acorrection coefficient based on a time since starting of the engine; andcorrecting the HC adsorption rate based on the determined correctioncoefficient.
 25. A method according to claim 24, wherein the step of HCadsorption rate determining includes: correction coefficient calculatingmeans for calculating a correction coefficient based on a time sincestarting of the engine; and correcting the HC adsorption rate based onthe determined correction coefficient.